/* Deque.java
** An instance of an implementing class represents a double-ended queue,
** or deque, which combines the capabilities of a stack and a queue.
** That is, in a deque, elements can be inserted at both the front and 
** the rear, and likewise removed from both the front and the rear. 
** Only the elements at the front and at the rear can be observed.
**
** Author: R. McCloskey
** Date: March 2019
*/
public interface Deque<T> {

   // observers
   // ---------


   /* Reports whether or not this deque is empty (i.e., has no elements in it).
   */
   boolean isEmpty();

   /* Reports whether or not this deque is full, which means that an 
   ** attempt to insert an item may fail.
   */
   boolean isFull();

   /* Reports how many items occupy this deque.
   */
   int sizeOf();

   /* Returns (a reference to) the item at the front of this deque.
   ** pre: !isEmpty()
   */
   T frontOf();

   /* Returns (a reference to) the item at the rear of this deque.
   ** pre: !isEmpty()
   */
   T rearOf();


   // mutators
   // --------

   /* Removes the item at the front of this deque.
   ** pre: !isEmpty()
   */
   void removeFront();

   /* Removes the item at the rear of this deque.
   ** pre: !isEmpty()
   */
   void removeRear();

   /* Inserts the specified item at the front of this deque.
   */
   void insertAtFront(T elem);

   /* Inserts the specified item at the rear of this deque.
   */
   void insertAtRear(T elem);
}